I doubt anyone started doping up just because of the extra effort of a few WBC games.
Truth it, there are probably a lot of guys in the league that still used substances that are banned/frowned upon. There are ways to hide it. These ways are also not 100% foolproof (it's a human error factor, really - guys make mistakes on their regimens), hence players occasionally getting caught.
For as long as human beings have competed in sporting events, they've looked for ways to cut corners and get the edge on opponents. They really doubled down on it when being an athlete became such a lucrative career. It's just human nature.